Record of plate boundary metamorphism during Gondwana breakup from Lu–Hf garnet geochronology of the Alpine Schist, New Zealand

Abstract

The Zealandia portion of the Pacific–Gondwana margin underwent widespread extension, fragmentation, separation and subsidence during the final stages in the breakup of Gondwana. Although these processes shaped the geology of New Zealand, their timing and the timing of subduction cessation in the region remain unclear. To investigate the timing of these processes, we used Lu–Hf garnet geochronology to date six samples of the Alpine Schist, which represents the metamorphic section of the former Zealandia margin. The garnet dates range from 97.3 ± 0.3 to 75.4 ± 1.3 Ma. Compositional zoning in garnet indicates that the spread in ages results from diachronous metamorphism in the upper plate at the Pacific–Gondwana margin, occurring concurrently with rifting of Zealandia from East Gondwana via opening of the Tasman Sea. Clear spatial trends in the timing of garnet growth throughout the Alpine Schist are absent, indicating that either regional age trends were offset by post‐metamorphic deformation, or that metamorphism did not result from a single regional heat source, and was instead driven by short‐duration, spatially dispersed processes such as episodic fluid‐fluxing or mechanical heating. Diachronous metamorphism of the Alpine Schist can be attributed to heat conduction from the rising upper mantle during widespread extension, progressive burial and heating of accretionary wedge sediments during ongoing horizontal shortening, or fluid‐fluxing sourced from a subducting and dehydrating Hikurangi Plateau. These results indicate that during separation of Zealandia from East Gondwana in the late Cretaceous, the crust at the Pacific–Gondwana margin remained hot, potentially facilitating the extensive thinning of the Zealandia lithosphere during this time.

Middle to Late Miocene Age for the End of Amphibolite‐Facies Mylonitization of the Alpine Schist, New Zealand: Implications for Onset of Transpression Across the Alpine Fault

“…(a) Temperature‐time (T‐t) diagram derived from age dating in currently exposed Alpine Schist mylonite (zircon fission track and (U‐Th)/He ages and apatite fission track ages (all italics) from Ring et al, 1 ) and all other data from this study 2 ; T‐t data indicate slow cooling between ~14–10 and ~2 Ma; solid blue line reflects time‐averaged cooling rates of ~10 °C/Myr. Inset shows extended T‐t path including Late Cretaceous/early Tertiary data from Vry et al () (Lu‐Hf on garnet), Scott et al () (U‐Pb on monazite), Briggs et al () (Lu‐Hf on garnet), and this study (Rb‐Sr age of 60 Ma from sample WAI). Stippled path virtually unconstrained showing cooling of Cretaceous high‐T rocks until onset of dextral transpression in Southern Alps in early Miocene.…”

“…Hence, the notion of a Late Cretaceous through Paleogene wider “Alpine Fault thermotectonic corridor” might be an artifact of Neogene transpression. In addition to the Late Cretaceous/Paleogene ages for high‐T metamorphism, there are also numerous Cretaceous/Paleogene low‐T thermochronologic ages across the South Island (Ring, Mortimer, et al, ; Tippett & Kamp, ) and Stewart Island (Reiners et al, ; Ring et al, ) (Table S6), which have hitherto hardly been considered in compilations for Late Cretaceous/Paleogene ages (Briggs et al, ; Cooper & Palin, ; Mortimer, ). However, when the lithosphere is deforming, there needs to be deformation compatibility, and ages for lower crustal high‐grade metamorphism are linked in some way with low‐T thermochronologic ages in the upper crust.…”

“…Despite this spatial correlation, the major metamorphic isograds and the dominant foliation are often correlated with a Mesozoic phase of metamorphism unrelated to formation of the present Southern Alps (Cooper, ; Craw et al, ; Findlay, ; Grapes, ; Grapes & Watanabe, , ; Holm et al, ). Recent age estimates for that metamorphism, and by inference for the timing of peak metamorphic conditions, range from 120–100 to ~68 Ma (Briggs et al, ; Chamberlain et al, ; Grapes, ; Mortimer & Cooper, ; Scott et al, ). Vry et al () obtained an imprecise Sm‐Nd age of 12 ± 37 Ma from a Cenozoic garnet rim (Zone 4 garnet in their nomenclature), demonstrating a Cenozoic tectonometamorphic overprint of the Alpine Schist, which they quantified at 580–620 °C and 1–1.15 GPa (equivalent to 37–45 km depth if an average mineral density of 2,700 kg/m 3 was assumed).…”

Accessory mineral petrochronology reveals 30 m.y. of partial melting during the separation of Zealandia from eastern Gondwana

“…The metamorphic record preserved in the Alpine Schist postdates the widely accepted estimate for the end of subduction along this section of the Gondwana margin (ca. 105 Ma), suggesting that metamorphism was synchronous with regional extension in Zealandia and the final stages of the breakup of Gondwana (Mortimer and Cooper, 2004;Vry et al, 2004;Ireland, 2013, 2015;Scott et al, 2015;Briggs et al, 2018). Peraluminous granitic pegmatite dikes and sills exposed in an ~5 km by ~27 km region of the Alpine Schist in the Mataketake Range region represent the only evidence of anatectic melting of the Alpine Schist (Wallace, 1974;Batt et al, 1999;Chamberlain et al, 1995).…”

“…Limited existing geochronology data suggest that the pegmatites range from 82 to 67 Ma (Chamberlain et al, 1995;Batt et al, 1999), postdating prograde garnet growth in this region by up to 15 m.y. (Vry et al, 2004;Briggs et al, 2018). However, it is unclear whether this age range represents multiple discrete pulses of melt, semicontinuous melt production, or Pb mobility by diffusion or recrystallization in the dated accessory minerals.…”